Speaker
Description
In the post-inflationary axion scenario, axions are produced by axion strings. A crucial parameter for the prediction of the axion dark matter mass is the length per unit volume of axion strings at the time of the QCD transition. This can be established only by numerical simulations, which are limited in volume and time, and must be extrapolated to physical values. After a brief history of simulations and the standard extrapolation method (“scaling”), I show results from our latest campaign of simulations. When analysed in the scaling framework, quite precise results are obtained: as measured in the string rest frame, the length density in the radiation era is $1.491(93)/(ct)ˆ2$. The importance for extrapolation of measuring the RMS velocity is emphasised. The much larger values for the string density obtained by other groups are due to extrapolations of the slow approach towards the inferred scaling value. Work in progress towards an axion mass prediction in the scaling framework is outlined.